1. Field of the Invention
The present invention relates to a method of and an apparatus for generating halftone dots to reproduce a halftone image.
2. Description of the Related Art
In printing process, halftone images of four primary colors, that is, yellow (Y), magenta (M), cyan (C), and black (K), are printed with respective inks on a printing sheet to produce a color print. Each halftone image includes a large number of halftone dots which represent density variation in the image, and each halftone dots includes a number of painted pixels. The larger the number of painted pixels in a halftone dot, the larger the halftone dot; the larger the halftone dots, the darker the halftone image becomes.
The halftone image on the film is generally produced with a scanner which includes a reading unit for reading an original color image to generate color separation image signals, and a recording unit for exposing a photosensitive film to reproduce halftone images on the film. The color separation image signals represent density variations of the respective four primary colors in the original color image. The recording unit compares each image signal with specific screen pattern data, which represent threshold values, to generate a control signal for each recording pixel as a result of the comparison, and exposes a halftone image on a photosensitive film while ON-OFF controlling a light beam in response to the control signal.
FIG. 1 shows an example of screen pattern data for one halftone dot region. In this specification, one halftone dot region corresponds to one halftone dot. In FIG. 1, one halftone dot region is made of 32.times.32 pixels to each which a threshold value ranging from 0 through 255 is assigned. The threshold value for each pixel is compared with the level of the image signal, and those pixels that have the threshold values less than the level of the image signal are to be exposed. For example, when the level of the image signal is 64 over the 32.times.32 pixels, the area shaded at its outline is exposed to make a halftone dot.
The superimposed four halftone images could cause the so-called "moire" in the color print. The orientation angles or screen angles of the four halftone images are therefore set at different values to prevent the moire. FIGS. 2(a) through 2(d) show dot arrangements where the screen angles 74 are set at 0 degrees, 15 degrees, 45 degrees, and 75 degrees, respectively.
The production of four dot arrangements with different screen angles .theta. can be executed by two typical methods, that is, the rational tangent method and the irrational tangent method. The tangent of each screen angle .theta. (tan.theta.) is set at a rational number in the rational tangent method, and it is set at an irrational number in the irrational tangent method.
FIG. 3 is a conceptive view illustrating an arrangement of screen pattern data for the screen angle .theta. of about fifteen degrees according to the rational tangent method. FIG. 3 includes four halftone dot regions. U denotes a primary scanning direction and V denotes a secondary scanning direction. According to the rational tangent method, screen pattern data is produced such that the screen pattern data is addressed along each primary scanning line. For example, when a first halftone dot region HD1 and a second halftone dot region HD2 are exposed along a scanning line SL, the screen pattern data along the scanning line SL are successively read out. The rational tangent method therefore requires four sets of screen pattern data for the four screen angles so that the screen pattern data is addressed along the primary scanning line at each screen angle.
It is sometimes required to change of a screen ruling, or the number of halftone dots per inch, of the halftone image. The screen ruling is changed by adjusting a diameter and an interval of luminous points on the photosensitive film in the rational tangent method. This, however, requires a complicated shuttle mechanism in the secondary scanning direction, and an expensive optical system which can change the diameter and interval of the luminous points.
The irrational tangent method, on the other hand, can change the screen ruling relatively easily. Screen pattern data used in the irrational tangent method includes threshold values which are assigned to pixels in one halftone dot region as shown in FIG. 1. FIG. 4 illustrates application of the screen pattern data in the irrational tangent method. In FIG. 4, X and Y coordinates axes denote addresses on a screen pattern memory, or a memory for storing screen pattern data, and U and V denote primary and secondary scanning directions, respectively. The coordinates (U, V) of an arbitrary point A in the U-V coordinate system are converted to the coordinates of the X-Y coordinate system as follows to make the address on the screen pattern memory: EQU Y=U*cos.theta.-V*sin.theta. (1a) EQU Y=U*sin.theta.+V*cos.theta. (1b)
where "*" denotes multiplication. When U=m*p and V=n*p, the equations (1a) and (1b) are rewritten as follows: EQU X=m*p*cos.theta.-n*p*sin.theta. (2a) EQU Y=m*p*sin.theta.+n*p*cos.theta. (2b)
where "m" and "n" are integers and "p" denotes a pitch of each recording pixel, or a side length of each recording pixel.
Since the subscanning coordinate value v is constant on each primary scanning line, the integer- "n" is also constant on each scanning line. Therefore, only the first terms of the equations (2a) and (2b) change in reading out the screen pattern memory along the scanning line. According to the equations (2a) and (2b), one-pixel progress in the primary scanning direction increases the X coordinate by p* cos.theta. and the Y coordinate by p* sin.theta.. The screen pattern data for one halftone dot region as shown in FIG. 1 is sufficient to produce halftone dot arrangements at four different screen angles accordingly. Furthermore, the screen ruling is also changeable by adjusting the pixel pitch p in the irrational tangent method.
FIG. 5 shows the relationship between the arrangement of recording pixels and the address on the screen pattern memory in the irrational tangent method. Here, recording pixels RP are disposed along scanning lines SL. The coordinates (U, V) of each recording pixel in the scanning coordinate system are converted to the coordinates (X, Y) in the address coordinate system to make the address on the screen pattern memory according to the above equations (2a) and (2b). In the example of FIG. 5, screen pattern data stored at the address (0, 0), (2, 1), and (3, 1) are read out when recording pixels RP on a first scanning line SL1 are to be exposed. In a similar manner, when recording pixels RP on a second scanning line SL2 are to be exposed, screen pattern data stored at the address (0, 2) and (1, 2) are read out. In the irrational tangent method, the address on the screen pattern memory does not go along the primary scanning direction U, and some addresses on the screen pattern memory are skipped. The plural addresses which are referred to in a certain halftone dot region is generally different from those in adjacent halftone dot regions.
Since part of the addresses on the screen pattern memory are skipped in the irrational tangent method, the size of a halftone dots to be exposed, that is, the number of pixels in the halftone dots, deviate even if those halftone dots are produced as a function of an image signal of a constant level. FIGS. 6(a) and 6(b) show examples of halftone dot generation as a function of an image signal with a fixed level. A polygonal region Rex defined by bold lines represents the address range of screen pattern data which are smaller than the level of the image signal, that is, the address range to be exposed. The address of the screen pattern data corresponds to the center of each recording pixel RP. Each recording pixels are defined by a square in FIGS. 6(a) and 6(b), and each pixel center is drawn by a closed circle or an open circle. The recording pixels whose center is included in the polygonal region Rex are determined to be exposed, while the recording pixels whose center is not included therein are determined not to be exposed. The centers of the exposed pixels are shown with closed circles, and the exposed pixels are shaded. Nine recording pixels RP are exposed in FIG. 6(a), and four pixels in FIG. 6(b).
As described above, in the irrational tangent method, variation in the relative positions of the addresses to be referred to causes fluctuations in area of the halftone dots. In other words, halftone dots which are produced as a function of an image signal of a constant level do not have the same area. The variation in the relative position increases the darkness in some image areas while increasing the lightness in other image areas, thus causing uneven and unstable image reproduction.
Several methods have been proposed to eliminate the unevenness of a reproduced image. There is a method which applies a table of random digits to the address. This method, however, deforms halftone dots. Another method shifts or distorts each halftone dot region at random. The second method, however, could not efficiently eliminate fluctuations in area. There is still another method which changes the distribution of light quantity of a light beam. The third method, however, requires a relatively complicated optical system for changing the light quantity of the light beam.